Removal of contaminants or impurities from a gas phase stream is a commonly encountered process in petroleum and natural gas processing. For example, many natural gas streams contain at least some carbon dioxide (CO2) in addition to the desired methane (CH4). Additionally, many refinery processes generate a gas phase output that includes a variety of species, such as CH4 and CO2, which are gases at standard temperature and pressure. Performing a separation on a gas phase stream containing CH4 can allow for removal of an impurity and/or diluent such as CO2 or nitrogen (N2) under controlled conditions. Such an impurity or diluent can then be directed to other processes, such as being directed to another use that reduces the loss of greenhouse gases to the environment. Accordingly, there is a continuing need for efficient processes and materials for selectively separating constituent gases from a stream of gases
Molecular sieve materials, both natural and synthetic, have been demonstrated in the past to be useful as adsorbents and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain molecular sieves, such as zeolites, aluminophosphates, and mesoporous materials, are ordered, porous crystalline materials having a definite crystalline structure as determined by X-ray diffraction (XRD). Within the crystalline molecular sieve material there are a large number of cavities which may be interconnected by a number of channels or pores. These cavities and pores are uniform in size within a specific molecular sieve material. Because the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of industrial processes.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the “Atlas of Zeolite Framework Types,” Sixth Revised Edition, Elsevier, 2007.
Zeolitic materials of the GME framework type are characterized by a three-dimensional channel system composed of 12-membered ring channels interconnected at right angles by a two-dimensional system of 8-membered ring channels. Gmelinite is a naturally occurring sodium-calcium zeolite of the GME framework type and has a typical composition of 8Na2O:4CaO:Al2O3:4SiO2:6H2O.
U.S. Pat. No. 4,061,717 reports the preparation of fault-free gmelinite using quaternary ammonium polymers as a structure directing agent.
U.S. Pat. No. 6,187,283 discloses the organotemplate-free hydrothermal conversion of low SiO2/Al2O3 mole ratio (SAR) Y-zeolite (FAU framework type, SAR=4.0-4.8) to gmelinite with strontium cations, under crystallization conditions including a temperature of 240° C. and a time of 14 days. Hydrothermal conversion of Y-zeolite with inorganic cations other than strontium produced materials other than synthetic gmelinite.
Conventional natural and synthetic gemlinite have a propensity to intergrow with chabazite or related zeolites, resulting in blockage of the 12-membered ring channel of the gmelinite structure and poor sorption properties resulting from a variety of possible intergrowths.
It has now been found that GME framework type zeolites substantially free of non-GME framework type material can be synthesized by organotemplate-free hydrothermal conversion of FAU framework type zeolites with sodium cations, under mild crystallization conditions.
The GME framework type zeolites disclosed herein can be suitable for selectively separating carbon dioxide (CO2) from multi-component gas feedstreams containing CO2 and at least one other gas component.